This application relates generally to balloon catheters, and more specifically to balloon catheter apparatuses and associated systems configured to emit electromagnetic waves in the form of microwave energy for treating disease.
Ablation catheters are well known in the art. The addition of a balloon to stabilize and secure the ablation catheter in a vessel has also been described. Ablation catheters are used in various medical procedures. Those skilled in the art will appreciate that ablation catheters can be used to treat atrial fibrillation, similar to the techniques described in U.S. Pat. No. 5,861,021, which is hereby incorporated by reference. Further, those skilled in the art can appreciate a balloon-stabilized thermal ablation catheter may be configured to be advanced through the vasculature to damage nerves associated with the nervous system thereby modulating the transmission of signals via the damaged nerves.
Specifically, balloon catheters containing a thermal energy emitter can be advanced via the vasculature and damage adjacent tissue on a cellular level, which in turn would modulate the transmission of certain signals originating from or carried through those damaged nerves. Renal sympathetic nerve activity is determined primarily by cardiovascular baroreceptors. Thus, decreases in perfusion pressure at the carotid sinus/aortic arch baroreceptors may reflexively raise renal sympathetic nerve activity and result in renal sympathetic nerve-dependent sodium retention. Conversely, volume expansion or mechanical stimulation of the atria results in a suppression of renal sympathetic nerve activity and a concomitant renal sympathetic nerve-mediated diuresis and natriuresis. Employing ablation balloon catheters to disrupt renal sympathetic nerve activity can thus treat cardiovascular and renal diseases where sympathetic nerve activity has negative physiologic consequences (e.g., hypertension, edema, congestive heart failure, etc.).
Conventional renal denervation techniques for treating cardiovascular hypertension involve one or more of a number of interventional methods to disrupt the renal sympathetic nerves. Examples of such techniques include thermal ablation, chemical ablation, and mechanical cutting devices incorporated in balloon catheters. However, the majority of the interventions are advanced through the vasculature to the renal artery, and because the renal artery is highly sensitive to thermal damage, use of such techniques may pose significant safety risks to the patient. One known risk associated with use of ablation catheters is that an energy emitter or probe with an elevated temperature may come into contact with the wall of the renal artery. If such an energy emitter or probe comes into contact with the renal artery wall, then the artery wall could be damaged and the lumen of the renal artery could narrow as a result of scarring and stenosis. Contact with a heated energy source or probe can cause unwanted tissue damage in other procedures outside renal denervation procedures, including cardiac tamponade, transient ischemic attacks, stroke, and death. Thus, known balloon catheter apparatuses have been ineffective at ablating targeted tissue while minimizing damage to healthy or untargeted tissue, or otherwise continue to pose safety risks to the patient.
Conventional balloon catheter systems that generate thermal energy using radio frequencies (“RF”) rely on conduction of the heat from monopolar electrodes embedded in or near a surface of the balloon wall to the inner surface of the artery. The frequencies that are used in these systems to generate thermal energy are selected according to the conductive characteristics of the vessel and surrounding tissue. Moreover, such systems require the use of a return electrode to provide a return path for the electrical current. Because high frequency current travels through the body from the balloon to the return electrode, sensory nerve damage can occur and would result in patient pain due to heating at the interface between the skin and the electrode. A need exists for a balloon catheter system that capable of more precise energy delivery to minimize any sensory nerve damage and associated pain.
Other RF conductive methods may use two-point contact electrodes, but such methods cause undue penetration of thermal energy into tissue surrounding the desired region of application. Regardless of whether such RF devices include one or two electrodes, the radio frequencies that are currently used to generate thermal energy are in the range of approximately tens of kilohertz to approximately one megahertz. Thermal energy arising from these frequencies results in ionic drag, rather than dielectric heating.
RF energy, when introduced to tissue, induces eddy currents in the tissue. These eddy currents are created by the changing electromagnetic field of the RF energy which acts on the electrons and ions in the tissue. The tissue has some finite resistance just like any conductive material, like an electrical wire. The tissue “resists” the flow of these eddy currents dissipating the energy as heat.
Prior balloon ablation catheters designed for use in renal denervation procedures have certain limitations. For example, WO 2012/061150 describes a balloon catheter system using static frequency selections of 915 MHz, 2.5 GHz, or 5.1 GHz, whereas an embodiment of a balloon catheter apparatus and associated system disclosed in the instant application uses phase shifting and/or frequency sweeping to optimize therapeutic results. Further, the static frequency antenna designs described in WO 2012/061150 have limited frequency bandwidth and thus require variable tuning between different patients due to the variability of the dielectric properties of patient tissues and fluids. As such, where existing designs may operate at sub-optimal tuning characteristics, an embodiment of a balloon catheter apparatus disclosed herein contains an emitter that is relatively broad-band and thus can readily adapt to variability in patient anatomy and body chemistry. Moreover, the phase shifting and/or sweeping feature disclosed herein allows for a balloon catheter system of the type herein described to deliver microwave energy to tissue at different radial depths, while avoiding dielectric-related thermal peaks in RF absorbing tissue or fluids. WO 2012/061150 further limits its target tissue at a depth less than 5 millimeter radially from the center of the renal artery. By contrast, an embodiment of a balloon catheter apparatus and associated system disclosed herein targets tissue in the range of about 0.5 mm to about 12 mm radially from the inner surface of the artery wall. Further, an embodiment of a balloon catheter apparatus and associated system disclosed herein is configured to operate in any vessel—not only the renal artery as described in WO 2012/061150.
Hypertension exacerbates nephropathy and other cardiovascular disease states. Chemical treatment of hypertension via angiotensin II receptor blockers and angiotensin-converting-enzyme inhibitors have limited efficacy and are associated with certain adverse events that limit the eligible patient population. Therefore, any treatment that can reduce hypertension and its associated consequences would be welcomed by the medical community.